As is known, TV transmission standards differ from one another by a considerable number of parameters, but may be categorized into two broad groups or families according to the, positive or negative, type of modulation of the reception signal.
Positive or negative modulation is the term used to define the difference between the positive and negative values of the video picture tonality. More specifically, an increase in the amplitude of the carrier waveform of a broadcast picture is referred to as positive modulation when an increase occurs in the brightness of a scene being taken in front of a TV camera. By converse, negative modulation occurs when an increase in the scene brightness produces a decrease in modulation amplitude.
A first group of TV standards comprehend the negative video modulation standards identified by the characters B, C, D, K, H, I, K1, M, and N.
A second group comprehend the positive video modulation standards identified by the characters A, C, E, and L.
A TV set intended for operation on signals from either of the above-referenced groups requires that it be equipped with an intermediate frequency demodulator which can operate on both positive and negative modulation signals. Furthermore, a properly operating demodulator will output a demodulated video signal with downward synchros (negative modulation) independently of the input signal modulation.
The prior art currently provides demodulators which are embodied as integrated circuits having a contact terminal pin commonly known as the standard selecting pin. By applying two different DC voltage levels to this pin, correct demodulation can be achieved for the signals in the aforesaid groups.
Such voltage levels are either adjusted by means of a control knob on the TV set or possibly a remote control unit associated therewith.
That prior approach, while substantially serving its purpose, has a drawback in that the adjustment of the TV standard is left at the discretion of the user who is not, however, able to tell the polarity of the video signal modulation.